The present invention concerns a crude (unpurified) toluene diisocyanate (TDI) modified in such a way that its use in the manufacture of polyurethane foams makes it possible to obtain foams having a high resilience and whose firmness, in particular, is improved.
The modified crude toluene diisocyanate is in the form of a prepolymer which has been made to undergo a heat aging process.
As is well known, TDI is prepared industrially by the phosgenation of one (preferably a mixture) toluene diamine or of one or a mixture of hydrochlorides of toluene diamines. During the course of this reaction, taking place in the presence of a solvent, some high molecular weight compounds are formed as a result of secondary reactions of the TDI's with themselves or with the toluene diamines at the same time as the TDI is being formed. These compounds principally consist of cyanamides, ureas, biurets, and dimerized and trimerized diisocyanates, as well as even more complex mixed compounds. They nevertheless have the common chemical characteristics of preserving one or more free and reactive --NCO groups. This phosgenation product obtained after elimination of the solvent contains a number of --NCO groups per kilogram falling between about 8.5 and 11.2.
Various advantages, both economical and technical; for instance a better resistance to bending fatigue and a greater ease of implementation, result from the use of a crude phosgenation product (from which the solvent has only been eliminated) as the polyisocyanate in the manufacture of polyurethane foams. The use of a crude TDI which is the undistilled phosgenation product or, as in the case of French Pat. No. 1,470,254, results from the mixture of purified TDI with the heavy residual compounds from the phosgenation, however, presents an important disadvantage due to its instability and poor keeping qualities. This drawback gives rise to the formation of secondary compounds which confer mediocre physical properties upon the foams being obtained, particularly as far as residual deformation following compression is concerned.
In order to remedy these drawbacks, various treatments of the crude TDI's have been proposed. In particular, French Pat. No. 2,253,771 can be cited, according to which the crude TDI is treated with a weak acid so as to convert the cyanamides. French Pat. No. 2,073,924 can also be cited, according to which part of the diisocyanate is trimerized in the presence of derivatives of guanidine.
Furthermore, it is known that the manufacture of flexible polyurethane foams having a high resilience, often defined by a sag factor above 2.5, remains difficult in spite of being well known. On one hand, stabilizers do not enter into the composition of the formulations; namely stabilizers such as surfactants resulting from alkylene oxide condensation with siloxanes, which are customarily used for the manufacture of flexible polyurethane foams, but only, if absolutely necessary, not very active silicone oils. On the other hand, the finished foam, above all in the case of continuous block casting, must not contain too large a quantity of closed cells which would result in a poor dimensional stability of the block.